タン, きも, い, まこと
gall bladder, courage, pluck, nerve
JLPT1 G7 S9 F1449
As per 旦# (long and flat) + 肉 flesh → a long, flat piece of fat or grease → lip gloss (← lip gloss made from fat/grease). However, 胆 was adopted as a Shinjitai (new character form) to replace 膽, according to which 胆 bears the meanings *gallbladder*; *courage*; *pluck* and *nerve*.
[Full Kanji Etymology List]